U.S. patent application number 10/240426 was filed with the patent office on 2003-09-04 for saw device and method for manufacture thereof.
Invention is credited to Inoue, Takashi, Matsuo, Satoshi, Namba, Akihiko.
Application Number | 20030164529 10/240426 |
Document ID | / |
Family ID | 26608492 |
Filed Date | 2003-09-04 |
United States Patent
Application |
20030164529 |
Kind Code |
A1 |
Inoue, Takashi ; et
al. |
September 4, 2003 |
Saw device and method for manufacture thereof
Abstract
A surface acoustic wave (SAW) device having improved moisture
resistance is provided. The device includes a piezoelectric
substrate, an interdigital transducer (IDT) electrode on a first
surface of the piezoelectric substrate, and a resin coating for
covering the IDT electrode. After a piece of resin material of the
resin coating is dipped in an amount of pure water as solvent
having a mass as 10 times great as the piece of resin material at
120.degree. C. under 2 atom pressure for twenty hours, a
concentration of chlorine ion in the solvent is not higher than 50
ppm
Inventors: |
Inoue, Takashi; (Osaka,
JP) ; Matsuo, Satoshi; (Osaka, JP) ; Namba,
Akihiko; (Osaka, JP) |
Correspondence
Address: |
Lawrence E Ashery
RatnerPrestia
One Westlakes Berwyn Suite 301
PO Box 980
Valley Forge
PA
19482-0980
US
|
Family ID: |
26608492 |
Appl. No.: |
10/240426 |
Filed: |
March 7, 2003 |
PCT Filed: |
January 24, 2002 |
PCT NO: |
PCT/JP02/00498 |
Current U.S.
Class: |
257/414 ;
257/416; 438/48; 438/50 |
Current CPC
Class: |
H03H 9/1092 20130101;
H01L 2924/181 20130101; H01L 2224/16225 20130101; H01L 2924/16235
20130101; H03H 9/1085 20130101; H03H 9/059 20130101; H03H 9/02929
20130101; H01L 2924/181 20130101; H01L 2924/00012 20130101 |
Class at
Publication: |
257/414 ; 438/48;
438/50; 257/416 |
International
Class: |
H01L 021/00; H01L
027/14; H01L 029/82 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2001 |
JP |
2001-21153 |
Jan 30, 2001 |
JP |
2001-21154 |
Claims
1. A surface acoustic wave (SAW) device comprising: a piezoelectric
substrate; an interdigital transducer (IDT) electrode on a first
surface of said piezoelectric substrate; and a resin coating for
covering said IDT electrode, wherein, after a piece of resin
material of said resin coating is dipped in an amount of pure water
as solvent having a mass as 10 times great as said piece of resin
material at 120.degree. C. under 2 atom pressure for twenty hours,
a concentration of chlorine ion in said solvent is not higher than
50 ppm.
2. A SAW device according to claim 1, wherein, after a piece of
resin material of said resin coating is dipped in an amount of pure
water as solvent having a mass as 10 times great as said piece of
resin material at 120.degree. C. under 2 atom pressure for twenty
hours, a concentration of bromine ion in said solvent is not higher
than 150 ppm
3. A SAW device according to claim 1, wherein said resin coating
covers a second surface and a side of said piezoelectric
substrate.
4. A SAW device according to claim 1, further comprising a resin
cover made of said resin material for covering over a periphery and
said IDT electrode.
5. A SAW device according to claim 1, wherein said IDT electrode
includes: a first layer containing aluminum on said piezoelectric
substrate; and a second layer containing at least one of Ti, Zr,
Nb, Ta, W, V, and Mn over said first layer.
6. A SAW device according to claim 5, wherein said second layer
contains aluminum.
7. A SAW device according to claim 5, wherein said IDT electrode
further includes a third layer between said first and second
layers, said third layer containing respective components of said
first and second layers.
8. A SAW device according to claim 1, wherein said IDT electrode
includes: a first layer containing at least one of Ti, Zr, Nb, Ta,
W, V, and Mn on said piezoelectric substrate; a second layer
containing aluminum over said first layer; and a third layer
containing at least one of Ti, Zr, Nb, Ta, W, V, and Mn over said
second layer.
9. A SAW device according to claim 8, wherein said third layer
contains aluminum.
10. A SAW device according to claim 8, wherein said IDT electrode
further includes: a fourth layer between said first and second
layers, said fourth layer containing respective components of said
first and second layers; and a fifth layer between said second and
third layers, said fifth layer containing respective components of
said second and third layers.
11. A SAW device according to claim 1, wherein said resin material
contains filler.
12. A method of manufacturing a surface acoustic wave (SAW) device,
comprising the steps of: forming an interdigital transducer (IDT)
electrode on a piezoelectric substrate; and covering over the IDT
electrode with a resin material, wherein, after a piece of resin
material of the resin coating is dipped in an amount of pure water
as solvent having a mass as 10 times great as the piece of resin
material at 120.degree. C. under 2 atom pressure for twenty hours,
a concentration of chlorine ion in the solvent is not higher than
50 ppm.
13. A method according to claim 12, wherein, after a piece of resin
material of the resin coating is dipped in an amount of pure water
as solvent having a mass as 10 times great as the piece of resin
material at 120.degree. C. under 2 atom pressure for twenty hours,
a concentration of bromine ion in the solvent is not higher than
150 ppm.
14. A method according to claim 12, wherein said step of forming
the IDT electrode comprises the sub-steps of: forming a first layer
containing aluminum on the piezoelectric substrate; and forming a
second layer containing at least one of Ti, Zr, Nb, Ta, W, V, and
Mn over the first layer.
15. A method according to claim 14, wherein said step of forming
the IDT electrodes further comprises the sub-step of heating the
first and second layers.
16. A method according to claim 15, wherein said sub-step of
heating the first and second layers includes the sub-step of
heating the first and second layers at a temperature ranging from
150 to 500.degree. C.
17. A method according to claim 12, wherein said step of forming
the IDT electrode comprises the sub-steps of: forming a first layer
containing at least one of Ti, Zr, Nb, Ta, W, V, and Mn on the
piezoelectric substrate; forming a second layer containing aluminum
over the first layer; and forming a third layer containing at least
one of Ti, Zr, Nb, Ta, W, V, and Mn over the second layer.
18. A method according to claim 17, wherein said step of forming
the IDT electrode further comprises the sub-step of heating the
first, second, and third layers.
19. A method according to claim 18, wherein said sub-step of
heating the first, second, and third layers includes the sub-step
of heating the first, second, and third layers at a temperature of
150 to 500.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a surface acoustic wave
(SAW) device for use in a radio communication apparatus or the like
and a method of manufacturing the device.
BACKGROUND OF THE INVENTION
[0002] A conventional surface acoustic wave (SAW) device is
disclosed in Japanese Patent Laid-Open No.11-163661. FIG. 10 is an
enlarged cross sectional view of a primary part of the SAW device.
The SAW device includes a piezoelectric substrate 1 made of
mono-crystal material, such as lithium tantalate, and interdigital
transducer (IDT) electrodes 2 on the surface of the piezoelectric
substrate 1. The IDT electrode 2 includes an upper layer 5 and a
lower layer 3 both made of aluminum alloy containing titanium and
an intermediate layer 4 made of Cu, Si, Ge, or the like for
preventing aluminum alloy crystalline particles from growing and
for reducing the local cell corrosion.
[0003] The IDT electrode 2 is made of the aluminum alloy at its
top. Therefore, when being exposed to high-moisture atmosphere for
a considerable length of time, the electrodes may cause aluminum in
it to be corroded, thus declining its properties.
SUMMARY OF THE INVENTION
[0004] A surface acoustic wave (SAW) device has a high moisture
resistance. The device includes a piezoelectric substrate, an
interdigital transducer (IDT) electrode on a first surface of the
piezoelectric substrate, and a resin coating for covering the IDT
electrode. After a piece of resin material of the resin coating is
dipped in an amount of pure water as solvent having a mass as 10
times great as the piece of resin material at 120.degree. C. under
2 atom pressure for twenty hours, a concentration of chlorine ion
in the solvent is not higher than 50 ppm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a cross sectional view of an SAW device according
to Embodiments 1 to 9 of the present invention.
[0006] FIG. 2 is an enlarged sectional view of a primary part of
the SAW device of Embodiments 1 to 5.
[0007] FIG. 3 is a cross sectional view of the SAW device of
Embodiments 1 to 8.
[0008] FIG. 4 is a cross sectional view of the SAW device of
Embodiments 1 to 8.
[0009] FIG. 5 is a cross sectional view of the SAW device of
Embodiments 1 to 8.
[0010] FIG. 6 is a cross sectional view of the SAW device of
Embodiment 6.
[0011] FIG. 7 is an enlarged cross sectional view of a primary part
of the SAW device of Embodiment 7.
[0012] FIG. 8 is an enlarged cross sectional view of a primary part
of the SAW device of Embodiment 8.
[0013] FIG. 9 is a cross sectional view of another SAW device of
Embodiment 8.
[0014] FIG. 10 is an enlarged cross sectional view of a primary
part of a conventional SAW device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] (Embodiment 1)
[0016] FIG. 1 is a cross sectional view of a surface acoustic wave
(SAW) device according to Embodiment 1 of the present invention.
FIG. 2 is an enlarged view of a primary part of the SAW device
shown in FIG. 1. FIGS. 3 to 5 are cross sectional views for
explaining steps of fabricating the SAW device shown in FIG. 1.
[0017] The SAW device of Embodiment 1 includes a SAW element 14.
The SAW element 14 includes a piezoelectric substrate 10 for
propagating a surface acoustic wave, interdigital transducer (IDT)
electrodes 11 for exciting the surface acoustic wave, reflector
electrodes 12 at both sides of the IDT electrodes 11, and pad
electrodes 13 connected electrically to the IDT electrodes 11. The
electrodes are provided on the surface of the piezoelectric
substrate 10. The piezoelectric substrate 10 of this embodiment is
made of lithium tantalate, and it may be made of any other
mono-crystalline piezoelectric material, such as lithium niobate,
quarts, potassium niobate, or langesite, or of a piezoelectric
thin-film, such as zinc oxide or aluminum nitride, provided on a
silicon, sapphire, or glass base. The thickness of the
piezoelectric substrate 10 is 350 .mu.m. The IDT electrode 11
includes a first metal film 111 and a second metal film 112
deposited on the first metal film 111, as shown in FIG. 2. The
first metal film 111 is made of aluminum or aluminum alloy (for
example, containing 99 wt. % of aluminum and 1 wt. % of copper)
having a thickness of 340 nm. The second metal film 112 is, but not
limited to, made of titanium in this embodiment, and may be made of
material selected from at least Ti, Zr, Nb, Ta, W, V, and Mn,
having a thickness of 10 nm. The reflector electrodes 12 and the
pad electrodes 13 have two-layer arrangement similar to the IDT
electrodes 11.
[0018] A circuit board 26 has upper surface electrodes 23 on its
upper surface and a terminal electrode 25 on its lower surface. The
circuit board 26 has through holes which are provided therein and
are filled with via electrodes 24 which connect between the surface
electrodes 23 and the terminal electrodes 25. The circuit board 26
is made of glass epoxy, Teflon, polyimide, their film, or ceramic,
such as alumina ceramic.
[0019] In the SAW element 14, the pad electrodes 13 and the surface
electrodes 23 are fixed on the upper surface of the circuit board
26 via projection electrodes 20. The projection electrodes 20 are
made of electrically conductive material, such as gold, tin, lead,
copper, silver, nickel, or their alloy.
[0020] A resin coating 21 covers a periphery of the SAW element 14
between the piezoelectric substrate 10 and the circuit board 26
except an oscillation space 22 for the IDT electrodes 11. As long
as desired characteristics are obtainable, the IDT electrodes 11
may partially touch the resin coating 21.
[0021] The resin coating 21 is made of epoxy resin, silicone resin,
urethane resin, acryl resin, or their mixture which have an
electrically insulating property and are non-detachable from the
piezoelectric substrate 10 enough to form the oscillation space 22.
The resin coating 21 may be doped with filler such as silicon
dioxide, magnesium oxide, or aluminum oxide, for adjusting a
thermal expansion factor of the coating. In Embodiment 1, the resin
coating 21 is made of epoxy resin material.
[0022] A procedure of fabricating the SAW device of Embodiment 1
will be described.
[0023] The first metal film 111 is formed almost entirely over a
large sheet-like piezoelectric substrate wafer 10A by sputtering.
Similarly, the second metal film 112 is formed on the film 111.
[0024] Then, a resist pattern is provided by photolithography on
the second metal film 112. The second metal film 112 and the first
metal film 111 are etched in this order by reactive ion etching
(RIE). Then, the resist pattern has been removed by ashing with
oxygen plasma, and thus, the IDT electrodes 11, the reflector
electrodes 12, and the pad electrodes 13 shown in FIG. 2 are
developed. While the sputtering is used in Embodiment 1 for forming
the first metal film 111 and the second metal film 112, the
sputtering may be replaced by electron beam vapor CVD or vacuum
heatup CVD. The RIE may be replaced by a known liftoff technique
for providing the electrodes 11 and 12. In the RIE for the second
metal film 112 and the first metal film 111, the etching action at
a right angle to side walls may adversely be effected as known
"side etching". This causes the side walls of the IDT electrodes 11
not to be perpendicular to the piezoelectric substrate 10, thus
causing the walls to be tapered or inverse tapered or to be shaped
concave or convex at the interface between the first metal film 111
and the second metal film 112. Any case may hardly decline the
advantage of the present invention.
[0025] Then, the projection electrodes 20 are formed on the pad
electrodes 13. Then, the piezoelectric substrate wafer 10 is
divided into pieces to be the SAW elements 14 shown in FIG. 3.
Since the projection electrodes 20 are joined to the surface
electrodes 23 on the circuit board 26 by ultrasonic waves, the SAW
element 14 is mounted on the circuit board 26. For the joining with
the ultrasonic waves, the surface electrodes 23 and the projection
electrodes 20 are made of gold.
[0026] Then, a sheet-like resin 21 is bonded over the SAW element
14 to a circuit board wafer 26A and is cured under pressure and
heat, so that the resin fills between the SAW element 14 and the
piezoelectric substrate 10. At this moment, a great care is taken
to prevent the resin coating 21 from physical contact with the IDT
electrodes 11. The construction of the resin coating 21 is not
limited to the described method but may be implemented by
application with a dispenser or the like. In the latter case, the
resin coating 21 is made of highly viscous resin material to
minimize the physical contact with the IDT electrodes 11.
[0027] Then, the circuit board wafer 26A is divided into pieces of
a desired shape which turn to the SAW devices shown in FIG. 5.
[0028] The resin coating 21 generally contains chlorine ions and
organic acid which are highly corrosive to the aluminum component
of the IDT electrodes 11. If the SAW device is left under a
high-temperature, high-moisture condition for a considerable length
of time, the resin coating 21 is infiltrated with water, which may
then accumulate adjacent to the IDT electrodes 11. During the
infiltration, the water is doped with chlorine ions released from
the resin coating 21 and then become acid at a location adjacent to
the IDT electrodes 11. This causes the IDT electrodes 11 to develop
contact corrosions by battery effect between two different metals
depending on the combination of the first metal film 111 and the
second metal film 112. As a result, the first metal film 121 may be
corroded at the sides, which are not covered with the second metal
film 121. As the IDT electrodes 11 is corroded, the SAW device will
be declined in the properties.
[0029] The water doped with chloride ions remains as liquid or
vapor and is corrosive to the IDT electrodes 11. Particularly under
a saturated vaporizing pressure, the water stays as liquid and
corrodes the IDT electrodes 11 more. Hence, the resin coating 21 is
preferably made not only of the above listed material but also of
material containing little chloride ion, which is highly corrosive
to the IDT electrodes 11.
[0030] The SAW device was subjected to a pressure cooker test (PCT)
which involved detention under a pressure of 2.03.times.10.sup.5
Pa, a moisture of 100%, and a temperature of 120.degree. C. Five
different resin materials applicable to the SAW device of the
embodiment were immersed in a pool of pure water having a mass 10
times greater than the resin materials and were left intact for
twenty hours under the above conditions. The resultant liquid was
analyzed by an ion chromatography. Concentration of chlorine ion is
shown in Table 1.
1 TABLE 1 Concentration of Chlorine Ion (ppm) Resin Material A 31
Resin Material B 51 Resin Material C 62 Resin Material D 70 Resin
Material E 105
[0031] The SAW devices made of five of the above resin material by
the described method were left under a pressure of
2.03.times.10.sup.5 Pa, a moisture of 100%, and a temperature of
120.degree. C. for forty hours, and were tested in their changes in
their insertion loss. Hundred SAW devices for each different resin
material were fabricated as SAW filters having a center frequency
of 942.5 MHz. Filters having insertion loss increasing by 0.3 dB or
more after the forty hours were regarded as defectives.
[0032] The result is shown in Table 2.
2 TABLE 2 Number of Defectives Resin Material A 0 Resin Material B
0 Resin Material C 1 Resin Material D 15 Resin Material E 55
[0033] As apparent from Table 2, no defective is found among the
filters of the resin materials A and B containing not more than 50
ppm of chlorine ion. However, defectives are found among the
filters of the resin materials C, D, and E containing more than 62
ppm of chlorine ion. If the resin coating 21 enclosing the SAW
element 14 is made of the resin material containing not more than
50 ppm of chlorine ion measured by PCT, the IDT electrodes 11 can
be prevented from corrosion.
[0034] After 1000 hours of a high-temperature, high-moisture test
under a temperature of 85.degree. C. and a moisture of 85%, no
defective among the SAW devices having the resin coating made of
the resin materials A, B, or C containing not more than 62 ppm of
chloride ion is found.
[0035] The IDT electrodes 11 has a two-layer structure. If the
resin material contains not more than 50 ppm of chlorine ion
measured by the PCT, even the SAW device including an IDT
electrodes of a single-layer structure made of aluminum or aluminum
alloy may exhibit moisture resistance. However, the SAW device
including the IDT electrodes having the two-layer structure
exhibits higher moisture resistance.
[0036] (Embodiment 2)
[0037] The structure of a surface acoustic wave (SAW) device of
Embodiment 2 is substantially identical to that of Embodiment 1 and
can be fabricated by the same method. The resin coating 21 of this
embodiment is however made of resin material which contain not more
than 50 ppm of chlorine ion examined by a pressure cooker test
(PCT) and contain different measurements of bromine ion examined by
the PCT.
[0038] Similarly to Embodiment 1, the resin materials were immersed
in a pool of pure water having a mass 10 times greater than the
resin materials and were left intact for twenty hours under
conditions of 120.degree. C. and 2.03.times.10.sup.5 Pa. The
resultant liquid were analyzed by an ion chromatography and their
measurements of the concentrations of the chlorine ion and the
bromine ion are shown in Table 3.
3 TABLE 3 Concentration of Concentration of Chlorine Ion (ppm)
Bromine Ion (ppm) Resin Material A 31 Not detected Resin Material F
33 280 Resin Material G 33 204 Resin Material H 33 182 Resin
Material I 33 153 Resin Material J 33 96
[0039] Hundred SAW devices for each material, made of six of the
resin material by the described method were left under a pressure
of 2.03.times.10.sup.5 Pa, a moisture of 100%, and a temperature of
120.degree. C. for forty hours, and were tested in their changes of
the insertion loss. The SAW devices of each different resin
material were fabricated as SAW filters having a center frequency
of 942.5 MHz. Filters having their insertion loss increasing by
greater than 0.3 dB or more after the forty hours were regarded as
defectives.
[0040] The result is shown in Table 4.
[0041] (Table 4)
4 Number of Defectives Resin Material A 0 Resin Material F 87 Resin
Material G 45 Resin Material H 23 Resin Material I 0 Resin Material
J 0
[0042] As apparent from Table 4, no defective is found among the
filters of the resin materials A, I, and J containing not more than
150 ppm of bromine ion. However, defectives are found among the
filters of the resin materials F, G, and H containing 80 ppm or
more of bromine ion. If the resin coating 21 enclosing the SAW
element 14 is made of the resin material containing not more than
50 ppm of chlorine ion and not more than 110 ppm of bromine ion
measured by the PCT, the IDT electrodes 11 can be further prevented
from corrosion.
[0043] After 1000 hours of a high-temperature, high-moisture test
under a temperature of 85.degree. C. and a moisture of 85%, no
defective is found among the SAW devices including the resin
coating of any of the resin materials A, B, and C containing not
more than 150 ppm of bromine ion.
[0044] (Embodiment 3)
[0045] A method of fabricating a surface acoustic wave (SAW) device
according to Embodiment 3 of the present invention will be
described.
[0046] The SAW device of this embodiment includes a second metal
film 112 of interdigital transducer (IDT) electrodes 11 made of
tantalum but not titanium. Other arrangements are identical to
those of Embodiment 1 as fabricated by the same method.
[0047] Hundred SAW devices were subjected to a pressure cooker test
(PCT) similarly to Embodiment 1. The result is shown in Table
5.
5 TABLE 5 Number of Defectives Resin Material A 0 Resin Material B
0 Resin Material C 0 Resin Material D 7 Resin Material E 32
[0048] As apparent from Table 5, no defective is found among the
devices of the resin materials A, B, C, and D containing not more
than 60 ppm of chlorine ion.
[0049] Similarly, the SAW devices of the resin materials A and F to
J described in Embodiment 2 were examined in influence of the
bromine ion. The result is shown in Table 6.
6 TABLE 6 Number of Defectives Resin Material A 0 Resin Material F
88 Resin Material G 35 Resin Material H 7 Resin Material I 0 Resin
Material J 0
[0050] As apparent from Table 6, no defective is found among the
SAW devices of the resin material containing not more than 150 ppm
of bromine ion. For the resin material F, the number of defectives
is almost equal to that of Embodiment 4. For the resin materials G
and H, the number of defectives in this embodiment is smaller than
that of Embodiment 2.
[0051] The SAW devices of Embodiment 3 have higher moisture
resistance than those of Embodiments 1 and 2.
[0052] It is known that the normal electrode potential of titanium
is closer to that of aluminum than to that of tantalum. In other
words, a difference of the normal electrode potential between
titanium and aluminum is smaller than that between tantalum and
aluminum. Therefore, it is told that the corrosion by contact
between two different metals more likely occurs for the second
metal film 112 made of tantalum rather than of titanium, and the
moisture resistance is more declined for a combination of tantalum
and aluminum.
[0053] However, the result of this embodiment exhibits a reverse of
what is told. For clarifying the advantage, an electrochemical test
was carried out for measuring the electrode potential difference
between the first metal film 111 and the second metal film 112 in
consideration of the environment where the SAW device were placed
in the test. With a reference electrode made of silver/silver
chloride, a sample electrode made of the metal for the first metal
film 111, and a counter electrode made of platinum in the test, the
device was immersed in electrolyte solution (described later) for
measuring a natural potential, i.e., a electrode potential having
the corroding current remain substantially constant. Then, the
metal of the second metal film 112 was examined similarly for
measuring the natural potential. A difference between respective
natural potentials of the two metals represents a difference
between respective electrode potentials under the test environment.
The smaller the difference, the more the corrosion by the contact
between the two metals is reduced.
[0054] The electrolyte solution was prepared by dissolving 30 to
300 ppm of NaCl as an electrolyte into pure water based on a test
result of the resin material 21 of the SAW device. The dose of NaCl
has a range since the concentration of the chloride ion is not
clearly found on the IDT electrodes 11 in the environmental test,
such as the PCT.
[0055] The test was carried out with the counter electrode made of
platinum and the sample electrode made of Al--Cu alloy or titanium.
The difference between respective potentials between the two
electrodes ranged from 35V to 0.45V.
[0056] In that case, the titanium electrode serves as a cathode,
and the Al--Cu electrode serves as an anode. Thus, the Al--Cu alloy
as the anode lost electrons, thus being corroded.
[0057] Similarly, the difference between respective potentials of
the counter electrode made of platinum and the sample electrode
made of Al--Cu alloy and tantalum was measured ranging from 0.25V
to 0.35V. The cathode was the tantalum electrode, while the anode
was the AL--Cu alloy electrode. The Al--Cu alloy as the anode was
corroded as losing electrons.
[0058] Then, the same test was carried out for measuring a
difference between respective potentials of the aluminum electrode
and the Al--Cu alloy electrode. It is found that the Al--Cu alloy
is shifted by 0.1 to 0.2V to a positive side (referred to as a
noble side, while a negative side is referred to as a base side
hereinafter) as compared with aluminum.
[0059] As apparent from above, when the SAW filter is located in a
corrosive condition, the electrode potentials of the electrodes
made of aluminum, Al--Cu alloy, tantalum, and titanium are more
noble in this order.
[0060] More specifically, the SAW filter of Embodiment 3 allows the
electrode potential difference between aluminum and titanium to
increase by 0.45 to 0.65V greater than a known value.
[0061] Therefore, if the first metal film 111 is made of Al--Cu
alloy, and if the second metal film 112 is made of tantalum, the
electrode potential difference between the metal films 111 and 112
can be smaller than that with aluminum and titanium, respectively,
hence reducing the corrosion of the IDT electrodes 11. In other
words, the SAW device of Embodiment 3 has higher moisture
resistance than that of Embodiment 1 and 2. The second metal film
112 may be made of material selected from Zr, Nb, W, V, and Mn. The
material having a small electrode potential difference between the
first metal film 111 and the second metal film 112 under the
condition where the SAW device operates prevents the electrode from
the corrosion, thus providing a SAW device having a high moisture
resistance increased.
[0062] (Embodiment 4)
[0063] A surface acoustic wave (SAW) device according to Embodiment
4 of the present invention is that of Embodiment 1 in which a
second metal film 112 made of any metal selected from Zr, Nb, Cr,
W, V, Mn, Pt, Au, and Pd. A resin coating 21 is made of either the
resin material B of Embodiment 1 or the resin material I of
Embodiment 2. The second metal film 112 of this embodiment is
selected from metal having higher moisture resistance itself and
serves as a cathode against aluminum and an aluminum alloy for a
first metal film 111.
[0064] Hundred SAW devices of Embodiment 4 were subjected to a
pressure cooker test (PCT) and a high-temperature, high-moisture
test similarly to Embodiment 1. No defective is found among the SAW
devices having the second metal film 112 made of Zr, Nb, W, V, Cr,
or Mn regardless of the type of the resin material. In the SAW
devices with the second metal film 112 made of Pt, Au, or Pd, an
electrode potential difference between the first metal film 111 and
the second metal film 112 is large, thus causing the first metal
film 111 to be corroded at its sides and declining their
characteristics.
[0065] As described, the SAW device including the resin 21
containing not more than 50 ppm of chloride ion and not more than
150 ppm of bromide ion exhibits high moisture resistance if the
second metal film 112 is made of any metal selected from Ti, Zr,
Nb, Ta, W, V, Cr, and Mn.
[0066] (Embodiment 5)
[0067] A surface acoustic wave (SAW) device according to Embodiment
5 of the present invention has a structure identical to that of
Embodiment 1, but different in the composition of the second metal
film 112 of the IDT electrode 11. More specifically, the second
metal film 112 of this embodiment is made of aluminum alloy
containing 30 wt. % of titanium.
[0068] The SAW device of this embodiment can have a smaller
electrode potential difference between the first metal film 111 and
the second metal film 112 by 0.15 to 0.25V than that of Embodiment
1. This reduces corrosion by contact between two different metals,
and improves the moisture resistance. The electrode potential
difference was measured similarly to Embodiment 3.
[0069] The mass of the second metal film 112 of this embodiment is
smaller by 72% and 19.5% than that of Embodiments 1 and 3,
respectively. As a result, if the IDT electrode 11 have a mass
equal to that of Embodiment 1 and 3, the first metal film 111 can
be thicker, thus decreasing electric resistance of the IDT
electrodes 11. Accordingly, the SAW device of this embodiment has a
reduced insertion loss.
[0070] The IDT electrode 11 can have the increased thickness, hence
improving the yield rate of the SAW devices. This is because
characteristics of the SAW device change due to the mass of the IDT
electrodes 11. For example, the smaller the specific gravity of the
metal in the IDT electrode 11, the more the thickness of the IDT
electrode 11 can increase. This can absorb an error of the
precision of deposition by sputtering, thus improving the yield
rate of the device.
[0071] According to Embodiment 5, the second metal film 112 is made
of, but not limited to, Al--Ti alloy containing 30 wt. % of
titanium. The second metal film 112 may be made of any other metal
containing different rate of Ti and aluminum alloy containing at
least one metal selected from Zr, Nb, Ta, W, V, Cr, and Mn with
equal effect.
[0072] (Embodiment 6)
[0073] FIG. 6 is an enlarged cross sectional view of a primary part
of a surface acoustic wave (SAW) device according to Embodiment 6
of the present invention. The SAW device of this embodiment has a
structure identical to that of Embodiment 1 except that the
construction of an interdigital transducer (IDT) electrodes 11 is
different.
[0074] The IDT electrode 11 of Embodiment 6 includes an alloy layer
114 between the first metal film 111 and the second metal film 112
and is made of alloy containing metal of the first metal film 111
and metal of the second metal film 112.
[0075] The first metal film 111 of a thickness of 340 nm is made of
aluminum alloy containing, for example, 99 wt. % of aluminum and 1
wt. % of copper. The second metal film 112 of 10 nm thick contains
titanium. The alloy layer 114 is made of alloy containing aluminum,
a primary ingredient of the first metal film 111, and titanium, a
primary ingredient of the second metal film 112.
[0076] A method of fabricating the SAW device of Embodiment 6 will
be explained.
[0077] The first metal film 111 made of aluminum or an aluminum
based material and the second metal film 112 made of titanium are
deposited in this order on the piezoelectric substrate 10 by
sputtering.
[0078] Then, a resist pattern is provided on the second metal film
112 by photolithography.
[0079] Then, the second metal film 112 and the first metal film 111
are etched in this order by a reactive ion etching (RIE). As the
resist pattern has been removed by ashing with oxygen plasma, the
IDT electrodes 11, the reflector electrodes 12, and the pad
electrodes 13 are formed, as shown in FIG. 6.
[0080] The piezoelectric substrate 10 on which the IDT electrodes
11 are patterned is heated up for diffusing aluminum in the first
metal film 111 and titanium in the second metal film 112. As a
result, the alloy layer 114 containing both aluminum and titanium
is developed between the first metal film 111 and the second metal
film 112.
[0081] The heating process is carried out commonly at a temperature
range of 150 to 500.degree. C. or preferably 150 to 350.degree. C.
The temperature for heating being lower than 150.degree. C. may
discourage the dispersion of aluminum and titanium. The temperature
exceeding 500.degree. C., which is close to a melting point of
aluminum of 660.degree. C., may hardly stabilize the resultant IDT
electrode 11 in its characteristics.
[0082] The heating process may be performed preferably under 150 to
350.degree. C. The temperature not higher than 350.degree. C. is
preferable for preventing the characteristics of the SAW device
from declining due to electric intensity breakage. It is noted that
the IDT electrode 11 may be free from such electric intensity
breakage even when being heated massively depending on a pattern of
the electrode.
[0083] The succeeding steps are identical to those of Embodiment
1.
[0084] The SAW devices of Embodiment 6, the SAW devices of
Embodiment 1, and comparative sample including an IDT electrode 11
made of a single layer of aluminum alloy doped with 1 wt. % of
copper were subjected to a pressure cooker test (PCT) similarly to
Embodiment 1 for examining their insertion losses.
[0085] The SAW devices of this embodiment, similarly to Embodiment
1, were provided as SAW filters having a center frequency of 942.5
MHz. A SAW filter having an insertion loss increasing by 0.3 dB or
more after forty hours was judged as a defective. 100 units for
each model were examined.
[0086] As a result, all the comparative examples are defective. No
defective is found among the SAW devices of Embodiments 1 and 6.
The SAW devices of Embodiment 6 have the insertion loss change less
than those of Embodiment 1 and more improved moisture
resistance.
[0087] The IDT electrodes were visually inspected with a scan-type
electron microscope. In the SAW devices of Embodiment 6, the first
metal film 111 made of aluminum based material is less corroded at
their side than those of Embodiment 1.
[0088] This will be explained in more detail.
[0089] The IDT electrodes 11 having a two-layer structure, such as
explained in Embodiment 1, produces a discontinuous electrode
potential difference between the first metal film 111 and the
second metal film 112. When acid water exists around the IDT
electrode 11, the first metal film 121 is more affected by
corrosion by contact between two metals. In the SAW device of
Embodiment 6, the intermediate alloy layer 114 between the first
metal film 121 and the second metal film 122 developed by thermal
diffusion of the two metals prevents the discontinuous electrode
potential difference from being produced, hence contributing to
attenuation of the corrosion by contact between two metals.
[0090] The second metal film 112 of Embodiment 6 is made of, but
not limited to, titanium. The film 112 may be made of any metal
selected from Zr, Nb, Ta, W, V, and Mn with equal effect.
[0091] The heating process of Embodiment 6 is carried out after the
IDT electrodes 11 is formed, and however, the process may be
carried out with equal effect after the deposition of the second
metal film 112. Thus, the heating process for the electrodes may be
shared with the curing process for the resin coating 21. In the
latter case, the heating temperature preferably ranges from 150 to
500.degree. C. This can prevent the SAW device from declined in its
characteristics due to electric intensity breakage. The electric
intensity breakage is preferably eliminated to proceed the heating
process for yielding the alloy layer 114 before the development of
the IDT electrodes 11 where the electrode potential difference
remains small on the piezoelectric substrate 10.
[0092] (Embodiment 7)
[0093] A surface acoustic wave (SAW) device according to Embodiment
7 of the present invention will be described referring to FIGS. 1,
3 to 5, and 7. Like components are denoted by like numerals as
those of Embodiment 1 and will be explained in no more detail.
[0094] The SAW device of Embodiment 7 is differentiated from that
of Embodiment 1 by a construction of an interdigital transducer
(IDT) electrodes 11. The IDT electrodes 11 of Embodiment 7 has a
three-layer structure. A third metal film 113 of a 10 nm thickness
containing at least one metal selected from Ti, Zr, Nb, Ta, W, V,
and Mn is provided on the piezoelectric substrate 10. The first
metal film 111 of a 340 nm thickness is made of aluminum alloy
containing 99 wt. % of aluminum and 1 wt. % of copper. The second
metal film 112 provided on the first metal film 111 of a 10 nm
thicjness contains at least one metal selected from Ti, Zr, Nb, Ta,
W, V, and Mn.
[0095] A method of fabricating the SAW device of Embodiment 7 will
be explained.
[0096] The third metal film 113, the first metal film 111, and the
second metal film 112 are deposited in this order almost entirely
on the piezoelectric substrate 10 by sputtering. Then, a resist
pattern is provided on the second metal film 112 by
photolithography. Then, the second metal film 112, the first metal
film 111, and the third metal film 113 are etched in this order by
a reactive ion etching (RIE). As the resist pattern has been
removed by ashing with oxygen plasma, the IDT electrode 11, a
reflector electrode 12, and a pad electrode 13 are formed, as shown
in FIG. 7. The succeeding steps are identical to those for
Embodiment 1.
[0097] The SAW devices of Embodiment 7, the SAW devices of
Embodiment 1, and comparative example having an IDT electrodes 11
including a single layer of aluminum alloy doped with 1 wt. % of
copper were subjected to a pressure cooker test (PCT) similarly to
Embodiment 1. The SAW devices, similarly to Embodiment 1, were
provided as SAW filters having a center frequency of 942.5 MHz. A
SAW filter having its insertion loss increasing by 0.3 dB or more
after forty hours was judged as a defective. 100 units for each
type were examined.
[0098] As a result, all the comparative examples are defective. No
defective is found among the SAW devices of both Embodiments 1 and
7. The SAW devices of Embodiment 7 have their insertion loss
changed less than those of Embodiment 1, and have more improved
moisture resistance. They were subjected to 1000 hours of a
high-temperature, high-moisture test under a temperature of
85.degree. C. and a moisture of 85%. This test proves that the SAW
devices of Embodiment 7 are improved in the moisture
resistance.
[0099] The IDT electrodes 11 were visually inspected with a
scan-type electron microscope. It is found that the IDT electrodes
of Embodiment 1 exhibit a higher degree of the corrosion on
aluminum at the sides of the first metal film 111 than those of
Embodiment 7.
[0100] This will be explained in more detail.
[0101] The third metal film 113 made of high-melting-point metal,
such as titanium, under the first metal layer 111 improves its
crystalline orientation. The crystalline orientation particularly
in the first metal film 111 of the SAW device of Embodiments 7 and
1 were examined by an X-ray diffraction technique. It is found that
the first metal film 111 of the SAW device of this embodiment is
oriented along (111) planes, while the first metal film 111 of the
SAW device of Embodiment 1 is polycrystalline having random
crystalline orientations.
[0102] That is, The first metal film 111 of the SAW device of
Embodiment 1 is polycrystalline, and additional ingredient, such as
copper, is separated as a crystalline particle. The separated
copper collaborates with aluminum of the main ingredient to locally
form a galvanic cell. As the IDT electrodes 11 is exposed to the
cell effect, aluminum, which is electrically more basic than
copper, may be corroded.
[0103] The SAW device of Embodiment 7 includes the third metal film
113 beneath the first metal film 111, thus having the enhanced
crystalline orientation in the first metal film 111. This can
suppress the corrosion of a crystalline particle due to local cell
effect, hence allowing the SAW device to have a higher moisture
resistance.
[0104] The third metal film 113 of Embodiment 7 is, but not limited
to, composed of a single layer. The film 113 may be composed of two
or more layers with equal success. For example, a third metal film
113 (or a second metal film 112), a first metal film 111, a third
metal film 113, a first metal film 111 . . . may be disposed one
over the other from the piezoelectric substrate 10 with the same
effect.
[0105] (Embodiment 8)
[0106] FIG. 8 is an enlarged cross sectional view of a primary part
of a surface acoustic wave (SAW) device according to Embodiment 8
of the present invention. Like components are denoted by like
numerals as those shown in FIG. 7 and will be explained in no more
detail.
[0107] The SAW device of Embodiment 8 is differentiated from that
of Embodiment 7 by that an interdigital transducer (IDT) electrode
11 provided on the piezoelectric substrate 10 includes an alloy
layer 115 between a first metal film 111 and a third metal film 113
and another alloy layer 114 between the first metal film 111 and
the second metal film 112. The layer 115 contains respective main
materials of the films 111 and 113, and the layer 113 contains
respective main materials of the films 111 and 112.
[0108] The first metal film 111 of 320 nm thickness is made of
aluminum alloy containing 99 wt. % of aluminum and 1 wt. % of
copper. The second metal film 112 and the third metal film 113 are
made of titanium of 10 nm thick. The alloy material of the alloy
layers 114 and 115 contains aluminum and titanium.
[0109] A method of fabricating the SAW device of Embodiment 8 will
be explained.
[0110] The third metal film 113 made of titanium, the first metal
film 111 made of aluminum or titanium, and the second metal film
112 made of titanium are disposed in this order on the
piezoelectric substrate 10 by sputtering.
[0111] Then, a resist pattern is provided on the second metal film
112 by photolithography. Then, the second metal film 112, the first
metal film 111, and the third metal film 113 are etched in this
order by a reactive ion etching (RIE). As the resist pattern has
been removed by ashing with oxygen plasma, the IDT electrode 11, a
reflector electrode 12, and a pad electrode 13 are formed, as shown
in FIG. 8.
[0112] The piezoelectric substrate 10 on which the IDT electrodes
11 are patterned is heated up for having aluminum and titanium
diffuse and for switching over between the first metal film 111 and
the second metal film 112 and between the first metal film 111 and
the third metal film 113. This provides the alloy layers 114 and
115 containing both aluminum and titanium. The heating process is
carried out commonly at a temperature of 150 to 500.degree. C., or
preferably 150 to 350.degree. C. The succeeding steps are identical
to those for Embodiment 7.
[0113] The SAW devices of Embodiment 8, the SAW devices of
Embodiments 6 and 7, and comparative examples having the IDT
electrodes 11 consisting of a single layer of aluminum alloy doped
with 1 wt. % of copper were subjected to a pressure cooker test
(PCT) similarly to Embodiment 1. More specifically, the SAW devices
were left under a pressure of 2.03.times.10.sup.5 Pa and at a
moisture of 100% for fourth hours, and were measured in their
insertion loss. The SAW devices were provided as SAW filters having
a center frequency of 942.5 MHz, similarly to Embodiment 1. A SAW
filter having its insertion loss increasing by 0.3 dB or more after
forty hours was judged as a defective. 100 units for each type were
examined.
[0114] As a result, all the comparative examples are defective. No
defective is found among the SAW devices of Embodiment 8 and both
Embodiments 6 and 7. The SAW devices of Embodiment 8 have their
insertion loss changed less than those of Embodiments 6 and 7, and
have more improved moisture resistance.
[0115] The IDT electrodes 11 were visually inspected using a
scan-type electron microscope. It is then found that the IDT
electrodes of the SAW devices of Embodiments 6 and 7 exhibit a
higher degree of the corrosion on aluminum at the sides of the
first metal film 111 made mainly of aluminum. The SAW device of
Embodiment 8 has no sign of the corrosion and is improved in the
moisture resistance.
[0116] In the foregoing embodiments, the SAW element 14 is tightly
covered at its periphery with the resin coating 21, while allowing
an oscillation space to be clearly formed for the IDT electrodes
11. As shown in FIG. 9, a resin cover 27 may be added over the
piezoelectric substrate 10 to protect the upper space above the IDT
electrodes 11 with equal success. If a resin material is used for
forming the space to enclose the SAW element 14 or is exposed to
the enclosing space, the concentration of chlorine ion measured by
the PCT is preferably not higher than 50 ppm. If the resin material
is doped with a filler, the resin coating 21 permits water from the
outside of the SAW device to move along a longer pass before the
water reach the oscillation space for the IDT electrodes 11. This
declines the water absorptivity of the resin coating 21.
Accordingly, as compared with no-filler doped resin material which
has the same concentration of the chloride or bromide ion, the
resin material doped with filler permits less amounts of water to
reach the IDT electrodes 11, hence contributing to the higher
moisture resistance of the SAW device.
INDUSTRIAL APPLICABILITY
[0117] The SAW device can be improved in moisture resistance, upon
including a resin coating made of specific resin material according
to the present invention for covering an interdigital transducer
electrode.
* * * * *